The invention relates to a method for joining two or more parts. Such a method can be used, for example, in the manufacture of a degasser for a liquid chromatograph for joining a gas-permeable membrane to a body receiving the liquid to be degassed.
In liquid chromatography, degassers are used to reduce the amount of gas dissolved in the solvents used. The solvents used in liquid chromatography are usually stored in contact with the surrounding air so that they are usually in a gas-saturated state. These dissolved gases adversely affect the chromatographic measurements.
U.S. Pat. No. 4,469,495 describes a liquid chromatography degasser for removing oxygen or other dissolved gases from a liquid used in liquid chromatography, wherein the liquid being degassed is passed through a spiral-shaped tube. This tube is made of a synthetic resin material, such as tetrafluoroethylene. U.S. Pat. No. 4,729,773 describes a device for degassing liquids in which the liquid is passed through a tube made of a fluor-resin which allows gases to pass through it while preventing the liquid from doing so. Degassers using flat membranes are known from U.S. Pat. No. 3,751,879 and from U.S. Pat. No. 3,735,562. Another degasser using a membrane is known from EP 718016.
Degassers using membranes typically comprise a cavity formed in a housing, through which the liquid to be degassed flows. On the side of the membrane distal from the cavity, vacuum is applied, thus forcing gases dissolved in the liquid through the membrane to the outside. The membrane has to be attached to the housing such that liquid cannot leak from the cavity to the outside through the connection between the membrane and the housing. A typical, cost-efficient method for joining the membrane and the housing is glueing.
Glueing, however, has several disadvantages. First, the glue or bonding agent may be dissolved by certain solvents used in chromatography, thus leading to leaks or complete destruction of the connection. Second, the glueing material may release unwanted substances which may be transported by the solvent stream in a liquid chromatograph to the detector where they interfere with the detection of the sample substances.
The problems mentioned in connection with a degasser can also occur with other parts in a liquid chromatograph which comprise a connection of two parts which come into contact with liquid, for example with solvent.
In view of the prior art, it is an object of the invention to provide a method for permanently joining solid parts in such a way that the region where the two parts are joined does not release unwanted substances when liquid comes into contact with said region, nor that the connection gets loose.
It is another object of the invention to provide a method for joining solid parts in a component of a liquid chromatograph which comes into contact with liquids.
It is a further object to provide such a method for manufacturing a liquid chromatography degasser.
It is a still further object to provide a method for joining solid parts which is cost efficient.
According to the present invention, these objects are achieved by a method comprising the following steps:
a) providing a foil between the two parts;
b) heating said foil to an elevated temperature such that it becomes soft;
c) providing a cooling step for cooling down the foil below the elevated temperature of step b).
The foil arranged between the two parts which are to be joined acts as a welding foil which is heated and then, after cooling down, provides for a permanent connection between the two parts. Preferably, the foil is locally heated to a temperature above the melting point. Preferably, at least one of the parts to be joined is prevented from thermal expansion during application of heat, for example by using a framing member surrounding the part.
According to an embodiment of the invention, one of the two joining partners is a thin membrane which may be as thin as 10 micrometers or thinner. This is particularly useful for manufacturing a liquid chromatography degasser comprising a thin gas-permeable membrane. A degasser which is preferably manufactured according to the method of the present invention is described in a co-pending European patent application entitled xe2x80x9cApparatus for Degassing Liquidsxe2x80x9d, filed on the same date and by the same applicant as the present application. The membrane can be made of a fluorinated synthetic material. In an embodiment of the invention, the membrane is stretched across a frame and then placed on the foil by corresponding movement of the frame. The frame may also provide the function of preventing thermal expansion of the body to which the membrane is connected. This embodiment thus provides a method for joining a thin membrane to a body by thermal xe2x80x9cweldingxe2x80x9d, even though the material of body would be unstable at the used temperatures.
According to another embodiment of the invention, one of the joining partners has a brim at least at one of its edges which is brought into contact with a thermode and pressed against the other joining partner. In this way, for example the housings of pressure transducers can be bonded to each other.
It is understood that the method of the invention can be used in a variety of applications. For example, it can be used for manufacturing lab-on-a-chip devices, wherein capillary structures are produced in a carrier substrate and a foil is attached to the carrier substrate with the method of the invention. The channel structures thus formed in the substrate, which is preferably made of a solvent resistant material, can be used for chemical analysis of fluids in the channels. Furthermore, the method of the invention can be used for joining foils, for example a polyether etherketone (PEEK ) foil with another PEEK foil. Also, the method of the invention can be used to connect valve membranes to a corresponding valve housing or valve seat. In accordance with a further aspect of the invention, a membrane can be directly bonded to a housing by a thermal treatment without an intermediate foil when at least one of the two joining partners becomes soft (starts to melt) by the application of heat. Examples of possible materials for the membrane are polyetheretherketone (PEEK) or polytetrafluoroethylene (PTFE), and the material of the housing may comprise PEEK or perfluoro alcoxy (PFA) or fluorinated ethylene propylene (FEP) .